Thatch is the dead grass and root tissue between the green vegetation and the soil. In layers of 1/2 inch or thicker, thatch can block light, water, and nutrients from reaching the roots. 

Common belief is that thatch is caused by a buildup of grass clippings, but this is not correct. Grass clippings are 90% water and begin to decompose almost immediately. Thatch is caused by a substance contained by grass called lignin. Lignin causes slow decomposition and this causes thatch to be a problem.

A little thatch is normal in yards, but when it creates a thick layer is when you run into trouble. If it begins to cause a problem, it can be removed in a process called dethatching. This can bedone with either a bamboo rake or a power machine. Once collected, thatch can be composted.